(1) Field of the Invention
This invention relates to a process of hot dip metallic coating of aluminum killed and low alloy steel strip and sheet material and more particularly to the preliminary treatment of the strip and sheet surfaces in a sulfur-containing atmosphere whereby to enhance the wettability thereof by molten coating metals such as zinc, zinc alloys, aluminum, aluminum alloys, and terne. Low alloy steels which may be treated by the process of the invention contain up to about 3% aluminum, up to about 1% titanium, up to about 2% silicon, or up to about 5% chromium, and mixtures thereof, with the remainder of the composition typical of carbon steel, as defined by Steel Products Manual, Carbon Sheet Steel, page 7 (May 1970), published by American Iron and Steel Institute. Aluminum killed steels include typical carbon steel as defined above containing from about 0.03% to about 0.06% acid-soluble aluminum.
(2) Description of the Prior Art
In the fluxless, hot dip metallic coating of steel strip and sheet, it is necessary to subject the strip and sheet surfaces to a preliminary treatment which provides a clean surface free of oxide scale which is readily wettable by the molten coating metal and to which the coating metal will adhere after solidification thereof. One of the principal types of anneal-in-line preliminary treatment, to which the present invention is applicable, is the so-called Selas process, a description of which is contained in U.S. Pat. No. 3,320,085, issued May 16, 1967 to C. A. Turner, Jr.
The Turner patent discloses a method of treating carbon steel strip and sheet material which comprises passing the material through a furnace heated to a temperature of at least about 2200.degree. F. (1205.degree. C.) by direct combustion of fuel and air therein, the furnace containing an atmosphere of gaseous products of combustion having no free oxygen and at least about 3% excess combustibles in the form of carbon monoxide and hydrogen, the residence time of the material being sufficient to cause it to reach a temperature of about 800.degree. to 1300.degree. F. (427.degree. to 705.degree. C.), while maintaining bright steel surfaces completely free from oxidation, withdrawing the material from the furnace while still surrounded by gaseous products of combustion, introducing the material directly into a reducing section having a hydrogen and nitrogen atmosphere, in which the material may be further heated from 800.degree. to 1700.degree. F. (427.degree. to 927.degree. C.) and/or cooled to approximately molten coating bath temperature, and then leading the material beneath the surface of the bath while surrounded by the hydrogen-nitrogen protective atmosphere.
U.S. Pat. No. 3,925,579 issued Dec. 9, 1975, to C. Flinchum et al, discloses a method of fluxless hot dip metallic coating of low alloy steel strip and sheet stock (as hereinabove defined) in which one or more alloying elements is present in an amount greater than the critical content thereof as hereinafter defined, wherein the surfaces of the stock are prepared for coating by heating to a temperature of about 593.degree. to about 913.degree. C. in an atmosphere oxidizing to iron whereby to produce a surface layer of iron oxide containing a uniform dispersion or solid solution of oxides of the alloying elements, followed by further heat treatment under conditions reducing to iron oxide. The method of this patent is applicable either to the Selas method, or to the so-called Sendzimir method of preliminary treatment (described in U.S. Pat. Nos. 2,110,893 and 2,197,622) which need not be described herein since the present invention is not practicable with the Sendzimir method. The method of the Flinchum et al patent is also applicable to aluminum killed steels which contain sufficient acid-soluble aluminum to cause poor adherence of the solidified coating metal when subjected to conventional preliminary treatment by the method disclosed in the Turner patent.
In all prior art processes for preliminary treatment of steel strip and sheet surfaces which are exposed to atmospheres of direct fired furnaces, including the methods of the above-mentioned Turner and Flinchum et al patents, it has been considered that the presence of even small amounts of sulfur, in the atmosphere would be highly deleterious. Accordingly, substantially sulfur-free fuel such as natural gas has been prescribed for use in such furnaces. However, natural gas shortages have made it necessary to consider alternative sources of fuel. In a steel mill having coke ovens, the use of coke oven gas as a fuel source would be an obvious choice except for the fact that raw coke oven gas ordinarily contains about 300 to 500 grains of sulfur per 100 cubic feet of gas, the sulfur being present primarily as hydrogen sulfide with a small amount of organic sulfur compounds. Although the gas can be easily scrubbed to a sulfur level of about 75 to 100 grains per 100 cubic feet, and with modern and more sophisticated equipment can be cleaned to a level of about 25 to 40 grains per 100 cubic feet, it has nevertheless been generally considered that the Selas-type preliminary treatment methods for in-line hot dip metallic coating could not tolerate even the lower sulfur levels of scrubbed coke oven gas. Accordingly, it was believed that curtailment of natural gas supply would force the shut-down of coating lines equipped with direct fired furnaces for preliminary treatment of steel strip and sheet material.